The wavelength division multiplexing (WDM) scheme is a scheme for transmitting multiple optical signals with different wavelengths through one optical fiber. A WDM optical network using the WDM scheme includes a central office (CO), optical network units (ONUs), and an optical distribution network for connecting the CO and the ONUs. Among various optical distribution networks, the most economic structure is a star configuration in which a remote node (RN) including an optical multiplexer/demultiplexer is disposed in an area adjacent to ONUs, a CO is connected to the RN by a single optical fiber, and then the RN is connected to the ONUs by optical fibers.
Now the most well-known WDM optical network has a configuration in which a wavelength band used for upstream WDM optical signals is separated and different from a wavelength band used for downstream WDM optical signals. In the most typical configuration of such a wavelength band-separated WDM optical network, one cyclic wavelength multiplexer/demultiplexer is disposed in a CO and an RN respectively, the CO-wavelength multiplexer/demultiplexer is connected to the RN-wavelength multiplexer/demultiplexer by a single optical fiber, and then each port of the wavelength multiplexer/demultiplexer is connected to an ONU. Here, an optical transceiver includes an optical filter for wavelength band separation, an optical transmitter, and an optical receiver.
However, the narrower a wavelength guard band, the more difficult it is to technically implement an optical filter for wavelength band separation. Also, since the wavelength band-separated WDM optical network requires a wavelength guard band, it has a disadvantage in that the wavelength band of upstream WDM optical signals and the wavelength band of downstream WDM optical signals are limited. Further, since the wavelength band-separated WDM optical network uses different wavelength bands for upstream/downstream WDM optical signals, it is complex in that upstream/downstream semiconductor light sources have different semiconductor medium compositions.
In order to remedy these shortcomings, a wavelength-reuse WDM optical network has been proposed. Although the wavelength-reuse WDM optical network is advantageous in that it uses the same semiconductor light source because the same wavelength is used for upstream/downstream WDM optical signals, it has a fundamental problem in that a WDM optical signal has a small extinction ratio and deterioration in the quality of a WDM optical signal is caused by Rayleigh backscattering and the like.